This invention relates to the preparation of the surfaces of ferrous metal parts for depositing thereon galvanized zinc coatings. The galvanizing process requires that the surface of the ferrous object be properly prepared before immersing in the zinc bath, to ensure a metallurgical bond between the ferrous surface and the zinc. Present industrial preparation techniques may be divided into two groups: processes using a flux, and processes using gaseous reduction of oxide coating on the surface, e.g., the Sendzimir process. The new concept for the preparation of the surface, herein described, is a variation on the fluxing technique. A "flux" as herein used is a substance that promotes the fusing of metals.
The two fluxing processes now commercially used are referred to as the "dry method" and the "wet method". These industry names are somewhat misleading in that the terms "dry" and "wet" are not actually descriptive of the techniques: in the "dry" method the flux is placed on the ferrous object by immersing the object in a hot aqueous zinc chloride-ammonium chloride flux solution prior to entering the zinc bath; while in the "wet" method the flux is placed on top of the zinc bath, and the ferrous object to be galvanized is passed through it into the zinc.
While the above-described processes have for many years been producing satisfactory galvanized products, there are several inherent drawbacks to them. For instance, the reaction by which the zinc chloride-ammonium chloride flux promotes the galvanizing process produces visible, voluminous gaseous emissions. While these emissions have not been found to be toxic, they are irritating to workers on continued, excessive exposure and, therefore, are coming under scrutiny by governmental agencies such as the U.S. Environmental Protection Agency and the Occupational Safety and Health Administration.
The fluxing processes require the immersion of a relatively cold object (typically below 95.degree. C., or 203.degree. F.) into the molten zinc bath (typically about 460.degree. C., or 860.degree. F.). The result is a lowering of the zinc temperature by, for example, 5.degree. to 11.degree. C., or 10.degree. to 20.degree. F. Thus, the molten zinc and the object must be reheated to the temperature at which galvanizing is to occur. This temperature recovery requires that the heat input to the galvanizing kettle be increased to raise the temperature in a relatively short time, thus decreasing the ability of the galvanizer to control the temperature of the bath within desired limits. The increased heat input through the walls of the kettle also shortens its life. Further, undue time must be spent for this recovery, thus decreasing the productivity of the bath. Finally, the inability to control closely immersion times removes one of the most important parameters that the galvanizer can use to regulate the weight of coatings.
In recent years the galvanizer has been called upon to handle ferrous metals of a wide variety of chemical compositions, and the various ferrous metals react with different rates in the galvanizing bath. This difference in behavior is especially crucial in the case of ferrous metals containing silicon. The normal galvanizing times and temperatures used for galvanizing rimmed steels (low silicon content) can produce excessively heavy, brittle coatings on silicon-containingferrous metals. This problem is particularly troublesome in objects consisting of several different types of ferrous metals welded or assembled together. Thus, to prevent this the galvanizer is required to control carefully the immersion time and temperature in the zinc bath. These controls are extremely difficult, if not impossible, to obtain with the present galvanizing processes.
Also, in recent years galvanizers have been called upon to handle more complex shapes. The shapes often involve parts of widely different cross-sections. When introduced into the molten zinc, the thinner sections heat more rapidly and galvanize to a greater coating thickness than the heavier parts. The coatings on the thin sections are then excessively thick when the coatings on the heavy parts meet the minimum thickness.